In conventional blasting operations, at construction sites, quarries, and the like, the location and placement of the charges of explosives is first determined and then blast holes are drilled. An explosive charge is then positioned within each of the blast holes with the size of the charge being dependent on the size of the blast required and the terrain or configuration of the site itself, and, thereafter, the charge is detonated. There are, in general, two principal types of explosives commonly used for blasting operations. The first type is hard powder explosives such as dynamite. The second type used is low sensitivity explosives such as an ammonium nitrate/diesel fuel mixture known as ANFO.
Dynamite is probably the most common explosive used in conventional blasting operations. This is because dynamite, and other similar hard powder explosives, can be easily measured and sized to ensure a high degree of control of the blasting. Additionally, dynamite, because it is substantially unaffected by moisture, can be used for blasting holes where there is a high moisture content as is typically present at most conventional blasting sites. The main drawback to such hard powder explosives is their expense. Hard powder explosives such as dynamite are very expensive to use, thus significantly increasing the costs of blasting. Additionally, dynamite is a Class A explosive, requiring strict compliance with a number of federal regulations governing its transport, storage, and use. Another drawback to the use of dynamite is that it is often difficult to position the dynamite sticks in a blasting hole. Often, roots and rock outcroppings project into the blasting hole and can catch or block the path of the dynamite sticks. The dynamite sticks also have a tendency to twist and catch the sides of the blasting hole, often causing blockage thereof. This prevents the charge from being seated properly for blasting and can even cause the sticks to be broken so that they cannot be completely exploded.
Low sensitivity explosives, such as ANFO, are easier to handle and are more stable than the hard explosives. Perhaps the biggest advantage of ANFO is, however, the cost. ANFO and other low sensitivity explosives are much less expensive to use than dynamite, and are Class C explosives that are not subject to the extensive federal regulations applicable to the Class A hard powder explosives. Additionally, ANFO can be packaged for loading and used with relative safety and ease by workers at the blast site itself. This provides a significant advantage as it is generally not possible to know how much explosive is required to blast a particular site until the site is drilled and inspected. Thus, when ANFO is used, workers regulate, package, and use the amounts of explosives they need on site after-determining the amount required.
The problem with ANFO and other such low sensitivity explosives is that they are much more limited in application than the hard explosives. Most blasting sites have a high amount of moisture present due to rain or water in the ground, which, when mixed with ANFO, dilutes the mixture and prevents its detonation. Thus, it has generally not been possible to use the cheaper conventional ANFO at most conventional blasting sites due to the presence of moisture. Explosive slurries containing an ANFO mixture are available for use in some "wet" blast holes. Such slurries are pumped into a blast hole, forcing the water out. However, such mixtures are fairly expensive, similar to hard powder explosives. Thus, the use of such slurries significantly increases the costs of blasting, much more so than using ANFO alone.
There have been numerous arrangements for packaging explosives in sealed containers for shipment and subsequent detonation. Examples of such containers are disclosed in U.S. Pat. Nos. 2,425,472, 4,023,474, 4,037,536, 4,383,484, and 4,485,741. The primary function of such containers is, however, safer storage and transport of hard powder explosives such as dynamite or trinitrotoluene (TNT), and not the protection of the explosive from moisture.
One of the biggest problems with such explosive packages is that they contain only a limited amount of explosive material sealed in the package by the explosive manufacturer. Since, as pointed out hereinbefore, it is generally not possible to know the amount of explosives that will be required to blast a particular site, it is not possible to know how much or how many explosive packages will be required before the job starts. If the amount of explosives is underestimated, blasting must be held up until more packages or canisters are shipped. Additionally, these explosive packages are expensive, and thus the costs of blasting are increased when additional canisters must be obtained. Even explosive cartridges such as disclosed in U.S. Pat. No. 3,082,689, which discloses an explosive package containing a fixed amount of an ammonium nitrate explosive (ANFO), are much more expensive to use than unpackaged ANFO, especially where a precise determination of the amount needed involves a fractional part of a package. ANFO is relatively simple and easy to mix, and, thus, the workers in the field can mix and use ANFO in precise amounts much more cheaply than buying such prepackaged cartridges.
Accordingly, it can be seen that it is desirable to provide a packaging system for explosives that enables relatively inexpensive, low sensitivity explosives to be easily and economically packaged in precise amounts at the blasting site in containers sealed against moisture for safe, reliable, and economic blasting operations.